1. Field of the Invention
The present invention relates in general to a ceramic pressure sensor, and more particularly to a ceramic pressure sensor suitably used for measuring the pressure of a fluid as in the cylinder bore of an internal combustion engine.
2. Discussion of the Prior Art
There is known a pessure sensor of a type in which strain detecting means is provided on a substrate such as a diaphragm which is deformable in response to a pressure applied thereof. The deformation of the substrate is detected as a variation in the electrical output of the strain detecting means, which represents the pressure applied to the substrate. Several forms of such a pressure sensor using a ceramic diaphragm and capable of operating in high temperature environments have been recently proposed, as disclosed in SAE Reports 820319 and 860474.
In the pressure sensor of the type using such a ceramic diaphragm, the diaphragm is disposed such that one of the opposite surfaces of the diaphragm is generally exposed directly to a measurement fluid existing in the external space, so that the pressure of the measurement fluid acts on the exposed surface of the diaphragm.
Also, the pressure sensor of the type indicated above is adapted such that a pressure sensing element including the ceramic diaphragm is fixedly accommodated within a cylindrical metallic housing. The pressure sensing element and the metallic housing are sealed by suitable sealing means, with respect to each other, so that the external measurement fluid in the external space will not enter the interior of the housing, in order to avoid an adverse influence of the measurement fluid introduced in the housing, on the operating response and measuring accuracy of the sensor.
Where the pressure sensor is used to measure the pressure within the cylinder bore of an internal combustion engine, for example, the pressure sensor is installed in a mounting hole formed in the lateral end portion of the cylinder head of a cylinder block of the engine, such that the sensor is exposed to a high-temperature atmosphere within the combustion chamber of the engine, through a suitable fluid passage communicating with the sensor and the combustion chamber, as disclosed in laid-open Publication No. 62-81036 of Japanese unexamined Utility Model Application. Therefore, the sealing means for establishing fluid tightness between the metallic housing and the pressure sensing element accommodated therein is exposed to a high temperature of the atmosphere from the combustion chamber. Thus, it has been practically impossible to use, as the sealing means, 0-rings made of resin or rubber materials which usually have high sealing capability.
In the light of the above, it is necessary to use a metallic 0-ring for sealing between the pressure sensing element and the metallic housing, where the sensing element is exposed to a high-temperature measurement fluid. Alternatively, a metallic washer is interposed between and in metallic contact with the housing and the sensing element, such that no gap is left between the two members. However, the use of such a metallic 0-ring or washer results in complicating the sealing structure. Moreover, the metallic 0-ring or washer is not capable of establishing a sufficient degree of fluid tightness of the metallic housing, where the pressure of the measurement fluid in the external space (e.g., combustion chamber of an internal combustion engine) is 100 atmospheric pressures or higher, for example. Accordingly, the use of the metallic seals suffers from various undesirable results. For example, where the pressure sensor is used to detect the pressure within a cylinder bore of an internal combustion engine operating under a high load condition, combustion gases and/or engine oils in the cylinder are forced into the interior of the housing of the pressure sensor, through the metallic seal. As a result, the strain detecting portion of the sensing element is stained and deteriorated, whereby the operating response and measuring accuracy of the sensor are adversely influenced.